Hyperthyroidism is one of the most common health problems in cats, especially in older kitties over the age of 10 years. It happens when the thyroid gland, which is responsible for regulating the speed of the body’s metabolism, malfunctions. This usually happens because of a non-cancerous tumor growing within the gland.

When the thyroid malfunctions, it can begin to put out too much thyroid hormone. This hormone, called thyroxine or T4, affects every cell in the body and has an impact on all major organ systems.

Too much T4 circulating in a cat’s body causes the cat’s metabolic rate to skyrocket, leading to serious problems. A metabolic rate that’s too high causes the heart to beat faster than normal (causing the heart wall becoming thickened and less flexible), blood pressure to rise (which over time causes damage to the cat’s kidneys, heart, eyes, and brain), and the body to burn too many calories. All this overstimulation also impacts the cat’s central nervous system, which can lead to anxiety, hyperactivity and behavioral problems.

Cats with hyperthyroidism can become seriously ill. Left untreated, it can lead to heart failure, kidney failure, starvation, and death. Fortunately, hyperthyroidism is treatable, and many cats who receive treatment can make a complete recovery.

How To Spot Signs Of Hyperthyroidism

Any older cat who is showing one or more of these symptoms should be suspected of having hyperthyroidism until proven otherwise:

Weight loss. This is usually first seen as loss of muscle mass along the cat’s backbone, and then generally gets worse over time, despite the cat eating normal (or more than normal) amounts of food.

Increased appetite. A higher metabolic rate causes the body to burn calories faster than the cat can take them in, which sends constant hunger signals to the brain. Cats with hyperthyroidism can have ravenous appetites, with some cats eating double their normal volume of food and still begging for more.

Dull, dry hair coat. The coat may also become matted from the cat not grooming as frequently because he or she doesn’t feel well.

Increased aggression, especially when being restrained. Hyperthyroid cats tend to feel panicked during restraint, especially at the veterinarian’s office, where they’re already stressed.

Anxiety. This can be severe, causing crying and restlessness. The cat may also have an anxious or frantic facial expression.

Increased thirst and urination.

Vomiting. This is more common in households with multiple cats, and usually results from rapid overeating.

Confusion, nighttime yowling and pacing. This is one of the most common behavioral symptoms of hyperthyroidism, and is often mistaken for dementia in older cats.

Decreased tolerance for heat. This is also related to a high metabolic rate, causing the cat to seek out cooler places to sit or sleep.

Diarrhea or softer-than-normal stool, with frequent trips to the litter box.

Increased breathing rate, panting, or difficulty breathing.

Faster-than-normal nail growth. Claws may also become thickened and more brittle.

Muscle twitching or tremors, usually occurring in severe cases.

The video below shows a cat with severe hyperthyroidism. This little girl is exhibiting many of the classic signs: older cat; extremely thin; dry, unkempt haircoat; nervous and agitated; and panting from stress.

Although these are by far the most frequently seen symptoms of hyperthyroidism in cats, a small percentage of cats may not show any signs at all. And to further complicate things, there is one form of hyperthyroidism, called apathetic hyperthyroidism, where cats seem to show the opposite signs – depression, low energy, weakness, and poor appetite. Less than 5% of hyperthyroidism cases are the apathetic version, but it’s good to be aware of it since it can still cause the same kind of permanent damage as traditional hyperthyroidism. That’s why testing is so important!

Diagnosing Hyperthyroidism

If you suspect that your cat may be suffering from hyperthyroidism, make an appointment with your veterinarian as soon as possible. The veterinarian will check your cat’s heart rate and blood pressure, and feel your cat’s throat to check for an enlarged thyroid gland.

Blood tests, including a complete blood count (CBC), chemistry panel, and a thyroid hormone test will then be run. In most cases, these simple tests can confirm a diagnosis of hyperthyroidism very quickly. However, up to 10% of hyperthyroid cats may have normal thyroid hormone levels in their bloodstream. In those cases, additional specialized testing may be necessary.

Treatment Options For Hyperthyroidism

There are currently 3 ways to treat hyperthyroidism in cats:

Medication

Tapazole (methimazole) is the most commonly used drug to treat hyperthyroid cats. This drug, given as a pill twice daily, inhibits the body’s production of T4.

Although it would seem at first glance that medication may be the simplest and least expensive approach, this is actually not true over the long term. Once a cat is on Tapazole, she must stay on it for the remainder of her life and have her blood routinely tested every 3-6 months to make sure she is stable. Between the cost of the medication and testing, expenses can add up quickly. The medication also has side effects that can be severe in some cats.

However, the biggest argument against using medication as treatment for hyperthyroidism is that it doesn’t cure the disease; it merely suppresses the symptoms. Although the level of circulating thyroid hormone may be reduced, damage to the heart is still occurring and these cats can still go on to develop heart disease and other complications.

Surgery

Surgery can be performed to completely remove the tumor within the thyroid that’s causing the problem. When this surgery is successfully done, it can solve the issue; however, this surgery can be very tricky.

There is no precise way of knowing exactly how much of the thyroid gland to remove; if not enough is taken out, the cat will remain hyperthyroid, and if too much is removed, the cat tips too far the other direction and can become hypothyroid. There is also a rather large risk of accidentally damaging the parathyroid glands, which are 4 small glands that surround the thyroid and control the body’s calcium levels.

Additionally, older cats with hyperthyroidism usually have heart disease and other organ problems, so they are at a significantly higher risk during anesthesia. Surgery can also be costly.

Radioiodine Therapy

This is the safest and most effective way to treat hyperthyroidism in cats. This therapy involves admitting the cat to a specialized treatment center, where one injection of radioactive iodine is given under the skin. The iodine is absorbed quickly into the bloodstream, and once it reaches the thyroid, it destroys the overactive portions of the thyroid gland. Afterwards, a stay of anywhere from 3-10 days is required by law until the level of radiation in the cat’s body returns to an acceptable level.

All symptoms of hyperthyroidism, including excessive thirst, can be completely eliminated with radioiodine treatment.

Although this treatment sounds intimidating, it offers a complete cure for cats with hyperthyroidism. Approximately 95% of cats are cured after one injection; for the remaining 5%,a second injection can be given at a later date, which is usually completely curative.

There are many benefits to radioiodine therapy. It requires no anesthesia, does not damage any other tissue or organs, is not painful, has no harmful side effects, and thyroid function returns to normal usually within one month. Studies published in 2006 found that cats who received this treatment lived twice as long as cats who were treated with medication alone.

Although the treatment sounds expensive, the cost of radioiodine therapy has come down over the years – to anywhere from $800 – $1,500, depending on where you live. When you consider the fact that this is a one-time curative treatment (versus a lifetime of medication and blood testing), and is safer and more effective than surgery, it can easily be the best and most cost-effective treatment.

A Fatal Disease That Doesn’t Have To Be

Hyperthyroidism is so common in cats that it should always be suspected in any middle-aged to older cat who is losing weight, even if the cat doesn’t exhibit any of the other classic symptoms. Unfortunately, cats in the early stages of hyperthyroidism usually eat well and appear to have lots of energy – which cat parents can misinterpret as a sign that their cat is healthy.

Since hyperthyroid cats have symptoms similar to other common feline diseases (like kidney disease or diabetes), sometimes thyroid problems go undiagnosed. It’s highly recommended that any middle-aged to older cat always have a thyroid test done as part of their annual exam to make sure that the thyroid is functioning properly. The earlier hyperthyroidism is detected, the sooner treatment can be started, and the greater the chance for a complete recovery and a long, happy life for your cat!

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Have you ever had a cat with hyperthyroidism, or known someone who has? Please share your story with us in the comments below!

Comments

Thank you, I’m glad you found it helpful! I’m a big proponent of thyroid testing in cats, since symptoms of hyperthyroidism can mimic so many other common kitty diseases (like diabetes and intestinal disorders). And since it’s not uncommon for a cat to have both diabetes and hyperthyroidism at the same time, it’s important to test so you can treat for both!

My cat has this and had 15mg of med daily which has now pushed him too far the other way and is now on 10mg Meds and another blood test to be done next week to see how it’s working but he is definitely losing more weight.he is 18 now he is very vocal night and day and has a heart murmer. I must admit I’m now worried this treatment isn’t working. Can’t bear the thought of losing him.he does tend to be more unsteady on his feet lately.im not expecting great news from the next bloods.also got to take a urine sample ????

Hi Sue, I’m so sorry to hear about your kitty. Sometimes getting cats and dogs regulated on the right amount of medications that they need can be a tricky trial-and-error. This happens with diabetic pets also when you’re trying to regulate their insulin levels. Hopefully your cat just needs to be on the right dose for him, and once he is maintaining at the correct dosage of medication he will feel much better!

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I'm an author, founder of the Good Pet Parent Blog, and former Registered Veterinary Technician. Through GoodPetParent.com, I share information on pet health, veterinary terminology, animal behavior and communication, training, and nutrition to help pet parents better understand and care for their animal companions. [More.]